Protecting legacy systems: risk assessment and security measures

What would happen if the backbone of your IT system failed? What if an outdated system turns into a security vulnerability in the middle of operations? The consequences of a core system going down are often severe: production interruptions, dissatisfied customers and contractual penalties.

Many organisations rely on IT systems that no longer reflect the current state of the art to run millions of business-critical processes every day. According to the study ‘Legacy Modernisation 2024‘, 72% of German companies are facing the challenge of modernising their outdated core business systems.

While these so-called legacy systems are functional and business-relevant, they entail considerable risks, including missing security updates, opaque dependencies, and outdated architectures, which make them a preferred target for cyberattacks.

However, a structured approach can ensure that even outdated systems are operated securely and reliably without the need for complete replacement. This article explains how modern risk assessment, threat modelling and middleware can be used to sustainably safeguard your legacy IT.

Securing legacy systems requires a methodical approach, which involves identifying, assessing and prioritising risks rather than just reacting selectively.

Determine which legacy systems are still active, including the software, interfaces and hardware used. If up-to-date documentation is missing, it can be reconstructed through interviews with experienced employees and technical reverse analyses.

Regular internal audits help verify compliance with your security policies. Complement these with external audits and penetration tests to avoid blind spots, particularly where outdated technologies make oversight more difficult.

Legacy systems can be scanned for known vulnerabilities using specialised tools. If official updates are unavailable, bespoke solutions may be needed to implement compensating controls.

What are realistic attack scenarios? What consequences would be faced by the company if a failure or data loss were to occur? And how likely is it to occur? Knowing the answers to these questions will help you take targeted action.

Over the years, permissions that are no longer required accumulate. Therefore, it is important to check them regularly and to rely on multi-factor authentication (MFA), role-based access control (RBAC) and the principle of least privilege. Just-in-time access and continuous authentication also increase security for particularly sensitive systems.

Continuous monitoring helps to detect unusual patterns and potential attacks at an early stage. Use external anomaly detection tools that independently analyse and report suspicious activity.

Technology only provides protection when used correctly. Provide role-specific training for operational technology personnel, administrators, and external service providers. In addition, conduct regular red team exercises tailored to legacy environments. A proven benchmark is two realistic scenarios per year.

Good security decisions are based on analysis, not intuition. Especially with legacy systems, the following applies: what is unknown cannot be protected. Often, there is a lack of up-to-date overviews of data flows, interfaces, and dependencies, all of which form the foundation of any sound security strategy.

The criticality of a vulnerability depends on its context.
A test system with open ports is less critical than an ERP backend processing sensitive customer data. Technical findings should therefore always be assessed in light of the following questions:

Practical experience shows that an effective analysis model should combine the following four perspectives:

Only the combination of these four viewpoints results in a robust risk analysis that can withstand scrutiny from executive management or supervisory authorities.

Many legacy systems do not support modern authentication methods, encryption or logging. This is precisely the area in which middleware demonstrates its strength.

As an independent protective layer, it sits between the system and its environment. Security controls can be implemented through it without the need to modify the system itself.

This decoupling transforms middleware into a protective layer for systems that cannot be adapted, making it a vital component in achieving sustainable security in evolving IT landscapes.

Modern middleware can do much more than just transmit data. It acts as a strategic security centre and intelligently controls protective measures.

In this way, the middleware becomes an active defender rather than just a ‘translator’ between systems.

The central question is not whether middleware provides protection, but where and to what extent. A well-founded risk assessment forms the basis for securing the right systems at the appropriate depth and allocating resources effectively.

Example: An old booking system processes sensitive customer data and is connected to a modern web application via an API, but it cannot verify authentication itself. This responsibility is assumed by middleware. It requests credentials, validates them, and only forwards the request if authentication is successful. At the same time, it can detect attacks on the interface and block them automatically, leaving the legacy system to continue operating unchanged.

Legacy systems are not inherently insecure; they simply require a different security approach. With structured risk assessment, targeted threat modelling and integrated middleware solutions, outdated systems can be effectively protected and operated in the long term.

Proactive action not only minimises risks, but also strengthens the resilience of the entire organisation.